© Alan SeallsAs far back as the 1940s, scientists have wondered about the causes of these clouds with gaps seemingly made by a giant hole punch.
Through this seeding process, they leave behind odd-shaped holes or channels in the clouds, which have long fascinated the public.
The key ingredient for developing these holes in the clouds: water droplets at subfreezing temperatures, below about 5 degrees Fahrenheit (-15 degrees Celsius). As air is cooled behind aircraft propellers or over jet wings, the water droplets freeze and drop toward Earth.
"Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby," says Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR) and lead author of a new study into the phenomenon.
"Just by flying an airplane through these clouds, you could produce as much precipitation as with seeding materials along the same path in the cloud."
Precipitation from planes may be particularly common in regions such as the Pacific Northwest and western Europe because of the frequent occurrence of cloud layers with supercooled droplets, Heymsfield says.
Comment: However... see this:
What's wrong with the sun?